The present invention relates to methods of epitaxial production of semiconductor silicon carbide.
Semiconductor silicon carbide in the form of p-type or n-type monocrystals or epitaxial layers finds application in the manufacture of a number of semiconductor instruments, such as different high-temperature diodes, semiconductor light sources, thermistors, high-temperature sensors of mechanical values, superhigh-frequency (SHF) devices, etc.
Known in the art are several methods of producing semiconductor silicon carbide. The more conventional methods involve the production of semiconductor silicon carbide by the sublimation method, i.e. by growing silicon carbide crystals from supersaturated vapors thereof which are formed when evaporating solid silicon carbide (vapor source), and with the vapor source being maintained at a higher temperature than the temperature in the crystal growing chamber. The source of the silicon carbide vapor is provided by a pre-synthesized polycrystalline silicon carbide (either of abrasive or of semiconductor purity) or by silicon carbide directly synthesized from silicon and carbon vapors during the process of crystal growth. These high-temperature sublimation methods of growing single crystals of silicon carbide are based on the Lely method (A. Lely, Ber. Dent. Gesellsch., 32, 229, 1955).
The Lely method teaches the growing of silicon carbide crystals on spontaneously appearing nuclei of SiC at temperatures of from 2450.degree. to 2700.degree. C. This method of growing crystals requires special furnaces with graphite heaters and crucibles for growing the crystals. After vacuum degassing at temperatures of up to 2000.degree. C., the furnace is filled with an inert gas, generally argon, to a pressure slightly exceeding atmospheric. Taking into consideration the insufficient area of the crucibles for growing crystals, the process of producing quality SiC crystals having a large surface area entails a sharp increase in the real volume of the crucible (which can be in the order of scores of liters) which is actually always filled with a finely dispersed or porous graphite heat insulation, i.e. the gas medium in the crucible is always mixed with the gas medium in the whole internal space of the furnace, with the result that impurities from the graphite insulation find their way into the crystal growing chamber of the crucible.
Thus the main disadvantage of the methods for growing SiC crystals based on the Lely technique is that the processes of crystal nucleation and growth are extremely difficult to control. The large real volume of the crucible, the need of using a large mass of porous graphite for the thermal insulation and for the structural members of the furnace, the extremely high temperatures required for carrying out the process in an inert gas media, which gases are difficult to purify of traces of gaseous impurities, of which involve all considerable difficulties in producing both pure and doped crystals. The fact that the process of crystal nucleation and growth is hard to control and the difficulties experienced in doping the crystals result in low yields of quality crystals. The high operating temperatures entail high energy consumption and excessive consumption of expensive graphite elements of the furnace. Thus, even low yields of crystals involve considerable specific costs per unit of production which render the process prohibitively inefficient.
A number of the above-mentioned disadvantages have been eliminated in a method wherein semiconductor silicon carbide is epitaxially produced by way of sublimation from a supersaturated vapor of SiC and using crystalline seeding with silicon carbide. The source of SiC vapor is a fine-grained silicon carbide disposed at a considerable distance, on the order of several centimeters, from the seed crystals which are oriented relative to the axis of the furnace at an angle selected by the trial-and-error method. The reduced operating pressure of the inert gas (down to 20 mm Hg) and the provision for the seed crystals permit the formation of crystalline layers at relatively low temperatures (under 2050.degree. C.). The method of sublimation growing of SiC, however, also possesses a number of disadvantages, namely:
1. The difficulty of creating a controlled temperature differential in the crystal growth zone (growing chamber) and consequently the impossibility of ensuring a uniform temperature for each crystal, especially with large quantities thereof, which results in different conditions for the growth of crystal layers.
2. Poor controllability of the SiC vapor flow, leading to large losses of SiC vapors which flow past the growth zones of the crystal layers, with resulting large losses of pure silicon carbide.
3. The impossibility of any further decrease the growth temperatures by further reduction of the inert gas pressure owing to the resulting graphitization of crystal seeds.
All of the above disadvantage bound to render the process uneconomical and ineffective.